I noticed a big drop in my Adsense CTR yesterday (11/22). I looked at a lot of things that I may have done including increasing traffic from Adwords, swapping out some web services that were not performing well on a few pages (out of hundreds), and I also checked for traffic changes due to the US holiday on Thursday. Nothing could account for the big drop off in CTR. The one thing that I did find different was that Google is now appending URL's for Google Analytics. Any traffic I receive from Google Adwords has something like following appended and it is causing Adsense ads not to load: "&gclid=CJWUovKXxIECFQpZPgodyFABCDEF" I have deleted the Analytics tracking from myAdwords account, I will post back the results.
I had a few domains set up with Analytics and it turns out I don't have much traffic from Adwords except for those few domains. I turned off Analytics and I still see the string appended. I have removed an Google image ad and now my text ads show up more reliably. I noticed the image ads always showed up but my text ads often left an empty white space. So it may be a combination of the Analytics URL and multiple ad blocks....I hope.